It has been found in a moment-resisting building having a structural steel framework, that most of the energy of an earthquake, or other extreme loading condition, is absorbed and dissipated, in or near the beam-to-column moment resisting joints of the building.
It is desirable to achieve greater strength, ductility and joint rotational capacity in beam-to-column moment resisting connections in order to make buildings less vulnerable to disastrous events. Greater connection strength, ductility and joint rotational capacity are particularly desirable in resisting sizeable moments in both the lateral and the vertical plane. That is, the beam-to-column moment-resisting connections in a steel frame building can be subjected to large rotational demands in the vertical plane due to interstory lateral building drift. Engineering analysis, design and full-scale specimen testing have determined that prior steel frame connection techniques can be substantially improved by strengthening the beam-to-column connection in a way which better resists and withstands the sizeable beam-to-column, joint rotations which are placed upon the beam and the column. That is, the beam-to-column connection must be a strong and ductile, moment-resisting connection.
Hollow tubular columns are structurally efficient members to use in a variety of building design applications (both structural and architectural), including moment frames. Hollow tubular columns include, but are not limited to, Hollow Structural Section (HSS) columns and built-up box columns. However traditional moment connections types that connect a wide flange (‘H’ section) beam to a hollow tubular column involve significantly different design considerations than does connecting a wide flange beam to a wide flange column. During loading conditions, the moments in the wide flange beams are resolved into concentrated forces at the beam flanges that must be transferred into the column. The main difference between a hollow tubular column and a wide flange column is how the forces from the beam flanges are transferred into the column webs to be resisted as shear. In a wide flange column, the web is located at the center of the column flange. In a hollow tubular column, the forces from the beam flanges applied to the column face must be transferred to the sidewalls of the column, which act as the webs of the column. For traditional moment connection types that connect a wide flange beam to a hollow tubular column, the side walls of the hollow tubular column facing the beams (“flange walls”) must structurally span between the other sidewalls (“webs”) of the column to transfer out-of-plane forces from the beam flanges to the column webs. Accordingly, for such traditional moment connection types, the thickness of the flange walls of the hollow tubular column becomes a critical consideration for the out of plane strength and stiffness of the flange walls.
Conventional methods of connecting a hollow tubular column to a wide flange beam must rely on technically uncertain and costly means to transfer significant moment forces to the webs of hollow tubular columns. These current methods are typically used in uniaxial moment frame applications. One such method is directly welding flanges of the wide flange beams to the flange wall faces of a hollow tubular column. This method is self-limiting when the applied moment approaches the full flexural strength of the beam because of the inherent out of plane flexibility of the flange wall thickness of the hollow tubular column. Therefore, the direct welding technique has limited capacity to transfer applied moment forces through out-of-plane bending and shear to the connecting webs of the hollow tubular column.
Another conventional method is through-plate connections wherein the hollow tubular column is cut in two places at each floor level to allow through-plates attached to the top and bottom flanges of the wide flange beam to pass through the column. These through-plates are welded along the full perimeter of the cut sections of the hollow tubular column on both top and bottom faces of each through-plate. These type of connections have proven to be both costly to fabricate and uncertain in their performance when subjected to violent earthquakes. For example, the connection may be inherently susceptible to out-of-plane punching shear failures in the through-plate due to cyclic tensile forces in the column.
Exterior diaphragm plate connections (also known as cut-out plates) are similar to the through-plate connections in that they use flange plates attached to the top and bottom flanges of the beam to transfer the moments. However, in the exterior diaphragm plate connection the hollow tubular column remains continuous and the top and bottom flange plates are made wider than the width of the hollow tubular column to allow for cut openings having a perimeter that surrounds and is attached to the full perimeter of the hollow tubular column. This connection is inherently difficult to fabricate and erect.
Interior diaphragm plate connections consist of shop welded plates that are cut to fit along the inside perimeter of the hollow tubular column, thereby stiffening the flange walls of the hollow tubular column and thus providing a strengthening means to transfer beam flange forces to the sidewall webs of the hollow tubular column. Top and bottom flanges of wide flange beam are directly welded to the flange wall faces of the column. The fabrication of this connection type is difficult because of precise fit up issues and difficulty in access for welding of interior diaphragm plates to inside faces of the hollow tubular column. The performance of this connection type is correspondingly uncertain.